1. Field of the Invention
The present invention relates to a liquid crystal display device having backlights.
2. Description of the Related Art
Liquid crystal display devices are in wide use for display devices of notebook personal computers and desktop personal computers since their power consumption is small and they require only small installation space. In recent years, liquid crystal display devices for television have been developed and are increasing their share of the market. Further, personal computers usable for viewing television broadcasting are being developed.
When they are used for television, improvement in display quality of moving images is especially demanded. As a method for improving display quality of moving images, a driving method of increasing response speed of liquid crystal, a method of on/off driving of backlights, and so on have been proposed. Devising on/off driving of backlights reduces blur of moving images occurring by so-called hold driving (disclosed in Japanese Unexamined Patent Application No. 2002-6815).
FIG. 1 shows a display system constituted of a typical liquid crystal display device having backlights and a control device for controlling the display of an image on the liquid crystal display device. The liquid crystal display device has a timing controller, a liquid crystal panel, and a backlight unit. The backlight unit is constituted of a plurality of backlights BL1 to BL4 and is disposed on a rear face of the liquid crystal panel. The timing controller receives a clock signal, a control signal, an image signal, and so on from the external control device and outputs a driving signal for display of an image on the liquid crystal panel, an image signal, and backlight control signals BLCON1 to BLCON4 for turning on and off the backlights BL1 to BL4 in sequence.
When the display system is a personal computer system, the control device is constituted of a control board mounted in a personal computer. This kind of control device has a function of converting image sources (video, DVD, television signals and so on) to signals with various resolutions and frequencies. The control device temporarily stores the image signal in a not-shown line memory or frame memory, so that it is capable of freely setting the output timing of the image signal and the control signal.
FIG. 2 shows one example of on/off control of the backlights in FIG. 1. In this example, the number of horizontal lines of the liquid crystal panel is 768. Therefore, each of the backlights BL1, BL2, BL3, BL4 is disposed for every 192 horizontal lines of the liquid crystal panel. One frame period for displaying one screen is constituted of a period for transmission of the image signal and a blank period BLANK that is a period up to the head of a subsequent frame from the transmission of the image signal. The control device selects not-shown 192 scanning lines in sequence to write the image signal to liquid crystal cells of each horizontal line. Further, the control device lights the backlights BL1, BL2, BL3, BL4 in sequence in synchronization with the write of the image signal to the scanning lines 1 to 192, 193 to 384, 385 to 576, 577 to 768. In this example, the backlight BL1 (or BL2, BL3, BL4) illuminates for a predetermined period immediately before an image signal of a subsequent frame is written to the corresponding scanning lines 1 to 192 (or 193 to 384, 385 to 576, 577 to 768).
The timing controller starts turning on the backlights BL1 to BL4 in synchronization with the driving of predetermined horizontal lines (L1 to L4) so that lighting periods ON of the backlights BL1 to BL4 are equal to one another. Specifically, the timing controller asserts the backlight control signal BLCON1 low in synchronization with the driving of the horizontal line L1. Similarly, the timing controller asserts the backlight control signals BLCON2 to BLCON4 low in synchronization with the driving of the horizontal lines L1, L2, L3, respectively. The backlights BL1 to BL4 illuminate during low level periods of the backlight control signals BLCON1 to BLCON4 respectively.
The backlights BL1 to BL4 are turned off immediately before a subsequent frame image is displayed as described above. Specifically, the timing controller negates the backlight control signals BLCON1 to BLCON4 high in order to turn off the backlights BL1 to BL4 when the driven horizontal lines are 1, 193, 385, 577, respectively. Thus, the turning-on timing and turning-off timing of each of the backlights BL1 to BL4 are determined by the horizontal line numbers set in advance.
When the display system is a personal computer system, the output timings of the image signal and the control signal are generally changeable by the design of a personal computer manufacturer, users options, and so on. For example, when the output repetition rate of the image signal is to be made higher, the blank period BLANK becomes longer even with the same one frame period.
FIG. 3 shows another example of the on/off control of the backlights in FIG. 1. One frame period is the same as that in FIG. 2, but due to the higher output repetition rate of the image signal, the blank period BLANK is longer than that in FIG. 2. Transition edges of the backlight control signals BLCON1 to BLCON4 are generated in synchronization with the driving timings of the horizontal lines set in advance. Consequently, lighting periods ON of the backlight control signals BLCON1, BLCON2 whose lighting periods ON overlap the blank period BLANK is longer. On the other hand, the lighting periods ON of the backlight control signals BLCON3, BLCON4 whose lighting periods ON do not overlap the blank period BLANK become shorter. As a result, an upper half of the liquid crystal panel facing the backlights BL1, BL2 has higher brightness and a lower half of the liquid crystal panel facing the backlights BL3, BL4 has lower brightness.
Further, the life of the backlights BL1 to BL4 depends on how long they illuminate. Therefore, the life of the backlights BL1, BL2 having longer lighting periods ON is shorter compared with the life of the backlights BL3, BL4 having shorter lighting periods ON. The backlights BL1 to BL4 are not replaceable individually since they are integrally structured as the backlight unit. Therefore, if any one of the backlights BL1 to BL4 is of no use, the entire backlight unit needs to be replaced. This means that the life of the backlight unit depends on the backlight having a longer lighting period ON, and the longer the period, the shorter the life. Further, brightness of the backlights BL1 to BL4 while they are on slightly lowers depending on the length of the lighting period ON thereof. Therefore, if the backlights BL1 to BL4 have different lighting periods ON, brightness of the backlights BL1 to BL4 while they are on will be differentiated through the long-term use.
FIG. 4 shows change in brightness of the backlights BL1 to BL4. The backlight BL (one of BL1 to BL4) turns on in synchronization with a falling edge of the backlight control signal BLCON (one of BLCON1 to BLCON4). At this time, there is a transition period TP1 before the brightness increases to its maximum value. Similarly, there is a transition period TP2 that is from a rising edge of the backlight control signal BLCON to an instant when the backlight BL turns off completely and brightness turns to zero.
The transition periods TP1, TP2 are characteristics peculiar to the backlight BL and they are constant irrespective of a refresh rate signifying one frame period and. Therefore, when the refresh rate is higher, a ratio of a stable period SP to a low level period (ON period in the drawing) of the backlight control signal BLCON is relatively low. In other words, even when the sum of the low level periods of the backlight control signal BLCON in a predetermined period is independent from the refresh rate and is constant, the brightness of the backlight BL (an integral value of the waveform in the drawing) is lower as the refresh rate is higher. As a result, for example, if a user of a personal computer manually sets a higher refresh rate, a screen of the liquid crystal display device becomes darker.
Further, a liquid crystal display device adopting a technique of on/off driving the backlights BL has a problem that the on/off period appears as flicker. The flicker is not conspicuous when an image in various colors displayed. But when an image with small motion is displayed (such as a case where a single color images over a plurality of frames is displayed on a part of a screen, the flicker is conspicuous.